Many emergency vehicles, including ambulances, police vehicles, and emergency fire apparatus vehicles, output visible warning signals through the use of flashing lights. The primary object of the flashing lights is to call attention to the vehicle in order to warn observers of an emergency situation. Consequently, the effectiveness of the warning system depends on how well the flashing lights are observed.
At the same time, lights attached to emergency vehicles are typically required to flash at a rate of between 60 and 240 flashes per minute (one to four hertz) as standardized by the Society of Automotive Engineers (SAE J-845, SAE J-595, SAE J-1318). Thus, any attempt to enhance the effectiveness of warning lights ordinarily must comply with these standards.
Strobe lights, or strobe tubes, which provide a very bright flash of light, are often employed for emergency signalling. However, strobe lights provide a relatively short flash of light, and thus have an undesirably long interval between flashes when flashed at the standardized flash rates.
One manner of enhancing the effectiveness of a strobe light-based emergency signalling system involves periodically energizing the strobe light with a packet of short, rapid driving pulses (i.e., "pulse packet") followed by a time duration wherein the light is not flashed. During the time that the strobe light is driven by the pulse packet, i.e., the "on" time, the light appears to an observer to be essentially continuous due to the persistence of the retina. The off time, when the packet is not sent, is provided so that the strobe and associated driving circuitry can recharge, and also so that the strobe light appears to be flashing at the standardized rate. Observers perceive the flash as light having an apparently longer duration and/or a greater intensity than similar strobe lights flashed by a single pulse with driving signals of comparable amplitudes.
Another attempt to improve signalling effectiveness involves increasing the frequency of the flash rate. However, increasing the frequency of the flash rate causes a decrease in the perceived light output intensity due to a reduction in the dwell time in the human eye. Consequently, as the flash rate increases, an increase in the intensity is needed to keep the perceived intensity constant. Nevertheless, as the flash rate increases, the ability to increase the intensity decreases as a result of a corresponding reduction in available recharging time of the strobe and driving circuitry. Thus an inherent tradeoff is present in the combined amount of visible activity. Alternatively, the brightness may be increased, but this comparably requires that the flash rate remain the same or be reduced.
Another method of enhancing signalling effectiveness involves alternating the flashes between two or more strobe lights, which may be filtered to produce lights of differing colors. With a single power supply, the strobes are driven such that one bulb or one set of bulbs is energized while the others are not. Multiple power supplies have been employed for indicating certain types of emergency situations, but they have not been synchronized in any particular manner.
In each of these above-described situations, there is no way to prevent a time interval between the energization bursts wherein none of the light sources are being energized. To a human observer, this time interval constitutes a "dark time" wherein no visible information is being broadcast from the vehicle, thereby lessening the potential effectiveness of the warning system.